With increasing down-scaling of integrated circuits and increasingly demanding requirements to speed of integrated circuits, transistors need to have higher drive currents with increasingly smaller dimensions. Fin Field-Effect Transistors (FinFET) were thus developed. FinFETs include vertical semiconductor fins above a substrate. The semiconductor fins are used to form source and drain regions, and channel regions between the source and drain regions. Shallow Trench Isolation (STI) regions are formed to define the semiconductor fins. The FinFETs also include gate stacks, which are formed on the sidewalls and the top surfaces of the semiconductor fins.
In the formation of the STI regions, various clean steps are performed. These steps cause recess of the top surfaces of STI regions. As a result of the clean steps, the center portions of the top surfaces of the STI regions are lower than edge portions of the top surfaces of the STI regions. The STI regions with such a surface profile can impact the performance of the FinFET.